1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more particularly a system in which the air-fuel ratio of the mixture to be supplied to the engine is controlled toward a target value in response to an output signal level of an oxygen concentration sensor.
2. Description of Background Information
Air-fuel ratio feedback control systems for an internal combustion engine are known wherein the oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor (referred to as O.sub.2 sensor hereinafter) and the air-fuel ratio of the mixture to be supplied to the engine is feedback controlled in response to an output signal level of the O.sub.2 sensor for the purification of the exhaust gas and improvements of the fuel economy. As an example of such an air-fuel ratio feedback control system, an air-intake side secondary air supply system for the feedback control is proposed, for example, in Japanese Patent Publication No. 55-3533 in which an open/close valve is disposed in an air intake side secondary air supply passage leading to a carburetor of the engine and a duty ratio of the open and close of the open/close valve, i.e. the supply of the air intake side secondary air, is feedback controlled in response to the output signal level of the O.sub.2 sensor.
In the usual air-fuel ratio feedback control systems, it is customary to use an O.sub.2 sensor whose output signal level is not proportional to the oxygen concentration in the exhaust gas. On the other hand, an O.sub.2 sensor has been developed recently whose output signal level varies generally in proportion to the oxygen concentration in the exhaust gas when the air-fuel ratio of the mixture to be supplied to the engine is leaner than a stoichiometric air-fuel ratio. For instance, an air-fuel ratio control system using an O.sub.2 sensor of this type for precisely controlling the air-fuel ratio toward a target air-fuel ratio in a lean air-fuel ratio is described in Japanese patent application laid open No. 58-59330.
In the air-fuel ratio feedback control system in which the air-fuel ratio is controlled by a feedback operation toward a target air-fuel ratio using such a "lean O.sub.2 sensor", the target air-fuel ratio is usually determined from a pressure within the intake pipe on the down stream side of the throttle valve, and the engine speed. However, with this type of air-fuel ratio feedback control system, the target air-fuel ratio was generally determined without considering load conditions of the vehicle. Therefore, the reduction of the fuel consumption of the engine under a running condition was not sufficient even with the control operation of the air-fuel ratio toward the target air-fuel ratio especially in an operational range of the vehicle in which the engine load is relatively light.
Further, when the change in the engine load occurs due to a down shift operation or an up shift operation of the transmission, the air-fuel ratio of the mixture to be supplied to the engine will deviate from the target air-fuel ratio. However, under such a condition, a delay of the air-fuel ratio control has been experienced in conventional systems because of a time period required for detecting the deviation of the air-fuel ratio of the mixture as a change in the oxygen concentration in the exhaust gas by means of the O.sub.2 sensor. This has been causing deterioration of the driveability of the vehicle.